1. Field of the Invention
The present invention relates to a lens apparatus which is used for a video camera, a digital camera, and a television camera and performs auto-focal point detection (AF) control based on an object image, and an image taking apparatus including the lens apparatus and a camera apparatus attachable to and detachable from the lens apparatus.
2. Description of the Related Art
In recent years, it is essential for an image taking apparatus such as a consumer digital camera or a consumer video camera to have an AF function.
A mainstream of such a kind of AF is an auto-focal point detection system for extracting a signal corresponding to the sharpness level of an object from an image pickup signal to perform its evaluation and for executing the focal point detection operation of an optical system based on the evaluation.
The demand for an increase in image quality is high and an interchangeable lens system is used in many cases. In the case of a camera system in which a lens is attachable to and detachable from a camera, there is a system in which AF processing is performed by the camera and a driving instruction is transmitted to the lens.
Examples of an instruction of a focus position which is communicated between the lens and the camera includes an instruction of, a physical position of a lens, that is, a lens position, and an instruction of an object distance. In the latter case of the instruction of the object distance, a range of the instruction is not changed by the optical magnification of the lens, so that suitable compatibility is obtained.
FIG. 7 is a structural block circuit diagram illustrating an AF system of an interchangeable lens type camera system in which a focus driving instruction and focus position information are communicated as described above. A lens apparatus 2 is attachable to and detachable from a camera apparatus 1. A signal obtained by photoelectric conversion in a CCD 3 of the camera apparatus 1 is processed into an image signal by an image signal generating portion 4.
An AF processing portion 5 extracts an edge component from the image signal. An object distance instructed value for driving a focus lens 12 of the lens apparatus 2 which is required to maximize the edge component is data-processed so as to be adapted to a communication format with the lens apparatus 2, and then output to the lens apparatus 2.
The lens apparatus 2 detects the object distance instructed value from a communication data series input to a camera interface portion 7. A distance/lens position conversion portion 8 converts the object distance instructed value into a focus lens position instructed value.
At this time, zoom position information of a zoom lens 9 is read by a zoom position detecting portion 10. The distance/lens position conversion portion 8 calculates (converts) the zoom position information and the object distance instructed value as the focus lens position instructed value using a conversion table.
The focus lens position instructed value converted in the distance/lens position conversion portion 8 is supplied to a focus driving portion 11.
The focus driving portion 11 generates a drive pulse for leading the focus lens 12 to a position specified by the focus lens position instructed value, thereby driving a focus motor 13.
An object distance value is calculated based on an output from the focus driving portion 11 and an output from the zoom position detecting portion 10 by a lens position-distance conversion portion 14 and input to the AF processing portion 5 through the camera interface portion 7 and a lens interface portion 6.
Then, the AF processing portion 5 checks a current position of the focus lens 12 and makes a determination if the focus lens 12 is being driven or stopped. The AF processing portion 5 obtains an object distance instructed value based on edge component information included in a current image signal and transmits the obtained object distance instructed value to the lens apparatus 2.
When such a series of operations is repeated, the focus lens 12 can be led to a focusing position.
In the conventional example, the distance instruction is transmitted from the camera to the lens while the camera determines the object distance during the AF operation, so there is the following problem.
That is, in the case of a rear focus lens, a moving distance value of the focus lens which covers an object distance range between a close side and an infinite side is changed corresponding to the zoom position.
For example, in the case of a tele-photo end, a driving amount of a step motor which corresponds to the moving distance of the focus lens 12 which is necessary to cover the range between the close side and the infinite side is 2,000 pulses. In contrast to this, in the case of a wide-angle end, the driving amount is 10 pulses.
Therefore, when the object distance value is calculated based on a pulse count, the object distance value is changed in 2,000 steps in the case of the tele-photo end. In the case of the wide-angle end, the object distance value is changed in mere 10 steps.
In other words, when the position of the close side is expressed by 0x0000 (16 bits) and the position of the infinite side is expressed by 0xFFFF (16 bits), the object distance value in the case of the tele-photo end varies by 0x20 for each pulse and the object distance value in the case of the wide-angle end varies by 0x1999 for each pulse.
Thus, a threshold value for determining a state of the focus lens 12 in the camera apparatus 1 should be changed according to the zoom position.